hide resolve[d]Repository() and ensureRepoExists() from public API

Git calls happen all over the place in Coin, and they involve heavy tooling around them. Three functions that are spread all over the place and account for thousands of the git calls, are

• resolveRepository()
• resolvedRepository()
• ensureRepoExists()

They all belong to class RepositoryManager. What would it take to have this class acting as a separate service, and invoking resolveRepository() and ensureRepoExists() only in __init__() while hiding it from externals?

This ticket is for coming up with a feasible and worthwhile refactoring for this mess, since it affects performance and readability of the code.